APPLICANT'S DESCRIPTION: Although transcriptional enhancers are essential for the expression of the immunoglobulin heavy chain (IgH) locus, they are also needed for the additional functions of V(D)J joining, V-region hypermutation and immunoglobulin (Ig) class switch recombination. This research is directed towards understanding the function of the enhancer that drives expression of the IgH locus in the channel catfish, a teleost fish and therefore lacking Ig class switching. Preliminary results suggest that the catfish IgH enhancer functions through a novel interaction of transcription factors bound to multiple interacting octamer and muE5 sites. The role of these sites in the function of the enhancer will be dissected by site-directed mutagenesis, and the function of the mutated enhancer will be assessed in transient expression assays upon transfection into catfish B-cells. Catfish muE5-binding factor(s) will be isolated and characterized. The interactions between Oct transcription factors, muE5binding factors and the DNA motifs that they recognize will be measured directly using the BiaCore 2000 instrument that utilizes surface plasmon resonance detection. Such studies will define whether or not cooperative binding reactions occur between the Oct and muE5-binding transcription factors. Gene expression driven by the interaction of octamer-binding (Oct) transcription factors with those binding to muE5 sites (typically bHLH or zinc-finger transcription factors) has not been reported in mammalian systems. Thus, these studies should shed light on novel' transcription factor interactions in Ig enhancers, and will also provide insight into how enhancer function has influenced the evolution of the IgH locus.